The present invention relates generally to an apparatus and method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to an apparatus and method for generating a substantially axisymmetric static magnetic field having long, straight contour lines in the resonance region.
It is well recognized that particles of an earth formation having non-zero nuclear spin magnetic moment, for example protons, have a tendency to align with a static magnetic field imposed on the formation. Such a magnetic field may be naturally generated, as is the case for the earth's magnetic field, BE. After an RF pulse applies a second oscillating magnetic field B1, transverse to BE, the protons will tend to precess about the BE vector with a characteristic resonance or Larmor frequency ωL which depends on the strength of the static magnetic field and the gyromagnetic ratio of the particle. Hydrogen nuclei (protons) precessing about a magnetic field BE of 0.5 gauss, for example, have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of the protons can generate a detectable oscillating voltage, known to those skilled in the art as a free induction decay or a spin echo, in a receiver coil. Hydrogen nuclei of water and hydrocarbons occurring in rock pores produce nuclear magnetic resonance (NMR) signals distinct from signals arising from other solids.
U.S. Pat. Nos. 4,717,878 issued to Taicher et al. and 5,055,787 issued to Kleinberg et al., describe NMR tools which employ permanent magnets to polarize hydrogen nuclei and generate a static magnetic field, B0, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. The atomic nuclei align with the applied field, B0, with a time constant of T1. After a period of polarization, the angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B1, perpendicular to the static field B0, at the Larmor frequency fL=γB0/2λ, where γ is the gyromagnetic ratio of the proton and B0 designates the static magnetic field strength. After termination of the RF pulse, the protons begin to precess in the plane perpendicular to B0. A sequence of refocusing RF pulses generates a sequence of spin-echoes which produce a detectable NMR signal in the antenna.
U.S. Pat. No. 5,557,201 describes a pulsed nuclear magnetism tool for formation evaluation while drilling. The tool includes a drill bit, drill string, and a pulsed nuclear magnetic resonance device housed within a drill collar made of nonmagnetic alloy. The tool includes a channel, within the drill string and pulsed NMR device, through which drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets, which are mounted with like poles facing each other, surrounding the channel, and an antenna coil mounted in an exterior surface of the drill string between the magnets. This tool is designed to resonate nuclei at a measurement region known to those skilled in the art as the saddle point.
Great Britain Pat. App. No. 2 310 500, published on Aug. 27, 1997, describes a measurement-while-drilling tool which includes a sensing apparatus for making nuclear magnetic resonance measurements of the earth formation. The NMR sensing apparatus is mounted in an annular recess formed into the exterior surface of the drill collar. In one embodiment, a flux closure is inserted into the recess. A magnet is disposed on the outer radial surface of the flux closure. The magnet is constructed from a plurality of radial segments which are magnetized radially outward from the longitudinal axis of the tool. The flux closure is required to provide suitable directional orientation of the magnetic field.
The tools developed in the prior art have disadvantages which limit their utility in nuclear magnetic resonance logging applications. Magnet designs of prior art tools do not simultaneously produce a highly axisymmetric static magnetic field with long straight contour lines in the resonance region of the formation under evaluation. These factors adversely affect the NMR measurement given the vertical motion of a wireline tool and the vertical and lateral motion of a logging-while-drilling tool.